The present invention relates to an image forming apparatus with a transfer belt device in which a transfer sheet for an image forming apparatus such as an electrophotographic copier is electrostatically supported and conveyed, a toner image on an image carrier is transferred onto the aforementioned transfer sheet by a physical means, and then the transfer sheet is conveyed to a fixing means.
FIG. 7 is a sectional view showing the structure of an embodiment of a conventional electrophotographic copier which is a conventional image forming apparatus. In the drawing, numeral 10 is a photoreceptor drum which is a drum-shaped image carrier, numeral 20 is a charger which charges the circumferential surface of the photoreceptor drum 10, numeral 30 is an exposure unit, numeral 13 is an image-exposure section, numeral 40 is a developing unit, numeral 50 is a transfer sheet supply unit, numeral 53 is a paper feed roller, P is a recording paper which is a transfer sheet, numeral 16 is a transfer unit which is a charger for transfer, numeral 17 is a separator which is a discharger for separation, by which paper is separated from the photoreceptor drum, numeral 80 is a fixing unit, numeral 90 is a cleaning unit, and numeral 56 is a conveyance belt which is a conveyance means for recording paper P.
Referring to the drawing, the operation of the aforementioned copier will be explained as follows. After the circumferential surface of the photoreceptor drum 10 has been uniformly charged by the charger 20, image exposure is conducted on the circumferential surface of the photoreceptor drum 10 by the exposure unit 30 at the image exposure section 13 so that an electrostatic latent image is formed. After that, the aforementioned latent image is developed by the developing unit 40 to be a visual toner image. This toner image is transferred onto recording paper P which is conveyed by a paper feed roller 53 of the transfer sheet supply unit 50 in synchronization with the aforementioned image formation. This transfer is conducted in such a manner that the recording paper P is charged from its back side by the transfer unit 16 to a polarity reverse to that of toner. After transfer has been completed, an AC high voltage is impressed by the separator 17 upon recording paper P so that it may be neutralized, thus the recording paper P can be separated from the photoreceptor drum 10. Separated recording paper P is conveyed by the conveyance belt 56 to the fixing unit 80, where the toner image formed on the recording paper is fixed, and then the recording paper is ejected onto a delivery tray provided outside the apparatus.
After the toner image has been transferred from the photoreceptor drum 10 onto the recording paper, the residual toner staying on the photoreceptor surface is removed by the cleaning unit 90 and the photoreceptor drum 10 stands by for the next copy cycle.
In the case of the aforementioned structure, however, it is necessary to adjust the discharging efficiency between the transfer unit 16 and the separator 17 to find out the optimum point for function for the purpose of ensuring the toner image transferability and separability, and yet the aforementioned discharging efficiency is greatly affected by the ambient conditions, and thus there is a problem of less latitude for reliability. The aforementioned transferability is affected by the electrical charge given to recording paper P and mechanical properties of recording paper P such as stiffness, surface smoothness and curl. These are largely fluctuated by the storage conditions of recording paper P and the environmental conditions such as temperature and humidity in a transfer operation, so that it is difficult to maintain the aforementioned factors in a good condition.
In the case where the diameter of the photoreceptor drum 10 is large, the force of the recording paper P wound around the drum 10 to revert to its initial flat shape is smaller than that in the case of the smaller diameter of the drum when recording paper P is transferred to the conveyance belt 56. Consequently, recording paper P onto which a toner image has been transferred, tends to stick to the surface of the photoreceptor drum 10 and thereby causes a jam, resulting in the lower papering property and lower reliability of the apparatus.
As technology to solve the aforementioned problems, a belt-shaped transfer and conveyance device shown in FIG. 8 has been disclosed in U.S Pat. No. 3,357,325.
In FIG. 8, numeral 10 is a photoreceptor drum which is a drum-shaped image carrier, numeral 70 is a transfer belt device which is a belt-shaped transfer and conveyance device, numeral 71 is a transfer belt which holds thereon an electric charge and attracts a transfer sheet, numerals 72, 73 are rollers around which a transfer belt 71 is supported and stretched and by which the transfer belt 71 is driven, numeral 60 is a pre-transfer paper charger which conducts corona-discharge in order to attract recording paper P onto the transfer belt 71 by an electrostatic force, numeral 16 is a transfer unit which is a charger for transferring a toner image from the photoreceptor drum 10 onto recording paper P, and numeral 160 is a transfer section. Due to the foregoing structure, recording paper P, while it is conveyed, can be strongly attracted to the surface of the transfer belt 71. Accordingly, transferability and separability can be improved in the transfer section.
The aforementioned transfer belt 70 is preferably used for a color image forming apparatus in which toner images are formed by superposing them on the photoreceptor drum 10 and transferred onto recording paper P at a time. In the conventional color image forming apparatus, toner images are superimposed on the photoreceptor drum 10, so that a plurality of developing units are provided around the circumferential portion of the photoreceptor drum 10, and therefore the diameter of the photoreceptor drum 10 is increased. Accordingly, the conventional electrostatic transfer separation system is not sufficient in separating a recording paper from the surface of the photoreceptor drum 10, so that more positive separability is required, and when toner images are superimposed, the amount of deposited toner is increased, so that a large amount of electrical charge for transfer is required, and therefore greater transfer-charge holding ability is required. From the aforementioned viewpoint, the aforementioned transfer belt device is superior to the conventional one.
In the image forming apparatus provided with the aforementioned transfer belt device, before the transfer of a toner image formed on the photoreceptor onto the transfer sheet, the transfer sheet is charged to the same polarity as that of toner on the photoreceptor, so that the transfer sheet is not attracted to the photoreceptor, and toner does not adhere to the transfer sheet unnecessarily. Accordingly, the transfer sheet is positively attracted to and held on the transfer belt and conveyed to the transfer section, and then the transfer sheet is charged to the polarity opposite to that of toner, so that the toner image or the photoreceptor is transferred and recorded on the transfer sheet.
There are following two types of charging as the aforementioned paper charging means for a transfer sheet. One of them is a charging type in which a charging electrode is provided and corona discharging is conducted. The other is a charging type in which voltage is impressed upon conductive material such as an elastic brush and then the conductive material is caused to come into contact with a transfer sheet. In the former type, the electrode is heavily soiled, and there is a tendency that the charging becomes uneven and insufficient due to uneven discharging. The later charging type utilizing an elastic brush is superior to the former on the aforementioned points.
However, in the case where the charging means utilizing the elastic brush is used, when a transparent sheet utilized for an over-head projector (OHP) is used as a transfer sheet, the sheet is made of a electrically insulating material, and therefore, the influence of the paper charging means remains when the image is transferred onto the sheet, and a longitudinal brush-shaped streak is made on the transferred toner image, resulting in remarkable deterioration of light permeability.